There are already known a variety of ink compositions for use in writing with a marking pen which form on an impervious writing surface, such as of enamel, resin or metal, generally called a white board, writings erasable by wiping lightly with felt, dry cloth or paper. These erasable ink compositions usually contain an additive called a separating agent together with an organic solvent, a pigment and a resin.
For example, there is described in Japanese Patent Publication No. 62-9149 an erasable ink composition which contains an aliphatic dibasic carboxylic acid diester and an aliphatic monobasic carboxylic acid ester together with a surfactant, a polyester resin and a saturated fatty acid triglyceride dissolved in an organic solvent.
Like the above exemplified erasable ink compositions, most of the conventional erasable ink compositions contain ketones or alcohols as an organic solvent, amoung which ketones are very toxic. Methanol is also toxic and smells bad. Ethanol is less toxic, but is expensive.
On the other hand, as described in Japanese Patent Application Laid-open No. 60-179478 and No. 60-219273, a water base erasble ink composition is also known which contains an emulsion of such a resin as polyvinyl acetate as a separating agent together with a dyestuff or a pigment as a colorant, and a surfactant. However, when writings are formed with such an ink composition on an impervious surface and dried, the resin emulsion forms a continuous adhesive film on the impervious surface, as is the case with an already known so-called strippable paint. Consequently the writing is not so easily separated or erased from the surface by light wiping. Moreover, when the writing is rather strongly wiped, it is separated as a continuous film from the surface. Namely, even when the writing is wiped only at a part thereof, it often happens that the entire writing is separated from the writing surface.
Thus a number of improvement of water base erasable ink composition for use in marking pens have been recently proposed. For example, as disclosed in Japanese Patent Application Laid-open No. 64-11174 and No. 1-188576, a water base erasable ink composition is known wherein a water soluble polyvinyl alcohol is used as a film-forming agent and an organosilicone surfactant as a separating agent. A further water base erasable ink composition is known in which a film-forming agent is composed either of a by nature water soluble resin or a resin which is by nature water-insoluble, but is made water-soluble by salt-formation with a base, such as water-solubilized styrene-maleic anhydride resin, together with a separating agent composed of an aqueous emulsion of an aliphatic carboxylic acd ester, a higher hydrocarbon or a higher alcohol, as described in Japanese Patent Application Laid-open No. 1-252681.
A still further water base erasable ink composition is known in which polyvinyl acetal resin is used as a film-forming resin and a polyalkylene glycol ester as a separating agent, as described in Japanese Patent Application Laid-open No. 2-117975.
However, the above mentioned prior art water base erasable ink compositions are not found to be sufficient yet in erasability. More specifically, the prior art water base erasable ink compositions deteriorate in erasability as time passes, or becomes less readily erasable depending upon ambient conditions, particularly upon humidity conditions, or upon a material of white board, particularly on a white board of resin such as of unsaturated polyester resin.